Cooling fans which are driven by brushless direct current motors (DC motor) are widely used in thermal management of a variety of electronic systems.
FIG. 2 schematically shows a control circuit of such a DC motor. The control circuit comprises a driver IC with winding coils L1, L2 of the DC motor connected thereto. A Hall effect sensor module (not shown) and a control module (not shown) are built into the driver IC. The Hall effect sensor is adapted to detect changes in the magnetic field of a permanent magnet of a rotor of the DC motor, and the driver IC amplifies the weak Hall voltage signal. Two output terminals O1 and O2 of the driver IC then alternatively output electrical current to the winding coils L1, L2 so that the winding coils L1, L2 are excited to drive the rotor to rotate.
Often, the rotational speed of the fan needs to be detected and regulated to achieve appropriate cooling fan speed. To detect the fan speed, the driver IC has a frequency generation (FG) output terminal predetermined as a rotation detection terminal. The output wave of the FG terminal is a series of square waves carrying the speed information that consists of a plurality of alternating high and low voltage signals. The high voltage shown by the output wave of the FG terminal is usually designed to be close or equal to that of the power source Vcc. A frequency of the square waves reflects the fan speed. During assembly of the motor, the FG terminal is connected to a speed detection circuit. This detection circuit is able to recognize the high or low voltage signal and therefore detect the fan speed.
In general, the FG terminal is reserved and exposed to facilitate assembly of the motor. However, during assembly thereof, the FG terminal may possibly contact with a human body. Discharge breakdown will likely occur through the FG terminal because the human body may carry considerable static electricity. Thus, the driver IC of the motor is prone to be damaged by the static electricity.
For the foregoing reasons, there is a need for a detection circuit which can avoid damage caused by static electricity of the human body.